I. Field of the Invention
The present invention relates to a method of determining the composition of a solid body or specimen, hereinafter also referred to as "target", which is scanned by a primary particle beam which causes secondary particles to be released which are detected and registered in dependence on the location of their release. More specifically the invention relates to methods of secondary ion mass spectroscopy (SIMS). The devices used for secondary ion mass spectroscopy are well known in the art and described, for example, in the periodical "The Review of Scientific Instruments", New York, vol. 42, no. 1, at pages 44 et seq.
II. Description of the Related Art
For the purpose of examining atomic or molecular concentrations in solid bodies, such as semiconductor materials, integrated circuits and the like, a primary particle beam is scanned over a field of a given size of the target. When the primary particles impact against the target, secondary particles, e.g. atoms or molecules, are released from it. A certain proportion of the secondary particles is ionised when they are knocked out of the target. The ions can be detected and analysed in a selective detector which is responsive to charged particles, e.g. in a mass selective detector comprising a Quadrupole mass filter and a photomultiplier dectector with an electronic counter. If the ionic concentrations are recorded in accordance with the location of their release the lateral distribution of the atomic or molecular concentrations in the target may be determined.
In an ionic beam scanning method disclosed in the Paper entitled "Time-of-Flight Effects in Quadrupole-Based Scanning Ion Microprobes", K. Wittmaack, SCANNING Vol. 3,2 (1980), an ionic beam is guided linearly over the field of the target to be scanned in a manner similar to the electron beam in a television tube. The secondary atoms or molecules are released, partially as secondary ions, at each point at which the primary ionic beam impacts on the test piece. Between the impact of the primary ionic beam on a surface element, hereinafter referred to as a "pixel", of the sample and the detection of the secondary particles in the detector a certain time elapses which is termed the "offset-time" .DELTA..tau.. The offset-time is dependent on the mass, energy and charge of the secondary ions and is naturally subject to a statistical distribution so that the offset-time .DELTA..tau. is associated with a mean time error .DELTA.t. This means that the reliability with which secondary ions detected in the detector can be associated with a certain pixel decreases with an increase in the scanning velocity. In order to achieve a high locational resolution the primary ionic beam is therefore guided over the test piece with a relatively low scanning velocity.
The known secondary ionic mass spectroscopy method, known in short as the ionic beam scanning method, is an extremely versatile method. It is possible to register atomic or molecular concentrations in dependence on the location of the impact of the primary ionic beam and thus to examine their spatial distribution--both laterally with respect to the direction of the primary ionic beam and also over a certain depth of the test piece by removing the upper atomic or molecular layers. With the primary ion beam it is also possible to obtain a mass spectrum of the atomic or molecular concentration by registering the secondary ions in dependence on the mass of the secondary ions by tuning the Quadrupole mass filter.
By far the most frequent use nowadays of the ionic beam scanning method resides in increasing the material removal rate to such an extent by the use of a high primary ionic beam density that in the scanned field a "crater" forms from which a further layer of material is removed on each successive scanning. This enables one to obtain a depth profile of atomic or molecular concentration in the test piece.
In very many solids to be examined, e.g. semiconductor materials, which are doped by diffusion or ion implantation the concentration of the element to be examined varies within a thin surface layer by several orders of magnitude. It is therefore important when registering the secondary ions not to distort the result by submerging the measurements of the secondary ions of low concentration removed at a certain depth in the secondary ions of higher concentration removed from a lesser depth at the edge of the crater and thus to render the measured values unreliable.
In order that such "crater edge effects" can be eliminated a "window" or gate is set within the crater, generally in the center of the crater, which covers the surface of the entire field scanned by the primary ionic beam, the edge of which window is spaced far enough from the crater edge to eliminate as substantially as possible the impact of the crater edge effects. Thus only those secondary ions are registered which have been detected during the period of time which corresponds to that during which the primary ionic beam was within the window.
When scanning linearly, the primary ionic beam crosses the edge of the window twice in each line which covers the window, namely once at time t.sub.1 on entering the window and a second time at time t.sub.3 when leaving the window. The offset-time .DELTA..tau., which depends on the mass, energy and charge of the secondary ions and in practice can only be measured with considerable difficulty, should thus be taken into account because at time t.sub.1 on entry into the window those secondary ions are firstly detected which have been released from the test piece at a position which corresponds to the time t.sub.1 -.DELTA..tau.. The same applies also to the time t.sub.3 of the exit from the window. Since the offset time .DELTA..tau. is associated with the time error .DELTA.t, the times t.sub.1 and t.sub.3 can only be determined with an accuracy .DELTA.t. The edges at the beginning and at the end of the window with respect to the direction of movement of the primary ionic beam are thus increasingly "blurred" with an increase of the time error .DELTA.t with respect to the period of dwell of the primary ionic beam on a pixel. In order to maintain the position and shape of the window in the middle of the crater a relatively low scanning velocity is thus necessary when scanning linearly.
It is thus an object of the invention to provide a method of determining the composition of a solid body of the type described above in which the position and shape of the window do not vary with a substantially increased scanning velocity.